In a recent article published in the journal Light Science & Applications, researchers from China and Italy have developed a highly sensitive dual-gas detection system using photoacoustic spectroscopy (PAS) with a single-longitudinal-mode (SLM) solid-state laser. By utilizing the tunability of the solid-state laser, the system aims to detect multiple gases simultaneously with exceptional sensitivity and accuracy.
a) Schematic diagram of the structure for the solid-state laser-based intra-cavity QEPAS sensor system. b) Physical image of the designed gas cell with a non-resonant off-beam configuration. Image Credit: https://www.nature.com/articles/s41377-024-01459-5
Background
Solid-state lasers have emerged as promising light sources for gas sensing applications due to their high-power output and stability. In this context, the study explores the use of an SLM solid-state laser for PAS, enabling precise detection of trace gases such as water vapor and ammonia.
The unique characteristics of the solid-state laser, including its wide tunability range and stable output power, make it an ideal candidate for dual-gas detection. By fine-tuning the laser wavelength and optimizing the experimental setup, the system aims to achieve superior sensitivity and selectivity in gas detection compared to conventional methods.
The Current Study
The solid-state laser development and gas detection were carried out following these steps:
- Solid-State Laser Construction: A fiber-coupled diode laser emitting at 795 nm served as the pump source for the solid-state laser. The gain material, an oriented Tm:YAP crystal, was chosen to generate a ~2 μm laser output. To achieve SLM operation, two uncoated etalons with varying thicknesses were put into the laser resonant cavity. The angular adjustment of the etalons and the crystal temperature was controlled using a homemade thermoelectric cooler (TEC) for coarse- and fine-tuning of the laser wavelength.
- Laser Characterization: The laser beam profile was analyzed using a CCD camera. The beam quality factor was determined using the knife-edge method. The spectrum emitting from the laser was measured with a wavelength meter and a Fabry–Perot interferometer to ensure single-mode operation and spectral purity.
- Experimental Setup: The laser beam was intensity-modulated using a mechanical chopper to facilitate gas detection in a resonant photoacoustic cell. The photoacoustic cell consisted of two buffers and a one-dimensional resonant tube with specific dimensions to minimize noise from gas flow. A capacitive microphone positioned at the center of the resonant tube detected the acoustic signals generated by gas absorption.
- Gas Detection and Analysis: Gas samples containing known concentrations of H2O and NH3 were flowed through the photoacoustic cell. The signal-to-noise ratio (SNR) was measured at different integration times to optimize detection sensitivity. The laser wavelength was locked to the absorption peaks of the target gases, and the photoacoustic signals were analyzed to determine the minimum detection limit (MDL) for H2O using the solid-state laser-based PAS system.
- Long-Term Stability Testing: The stability of the laser source was assessed by continuously monitoring the wavelength and output power for 30 minutes. The standard deviations of 0.9 ppm and 1.1 % for wavelength and power output, respectively.
Results and Discussion
Gas Detection Performance
The solid-state laser-based photoacoustic spectroscopy (PAS) system exhibited exceptional sensitivity in detecting trace gases, particularly H2O and NH3. By precisely tuning the laser wavelength to match the absorption peaks of the target gases, the system achieved a high SNR and superior MDL for H2O. The optimized experimental parameters, including the resonance frequency and bandwidth of the photoacoustic cell, contributed to the system's superior gas detection performance.
Dual-Gas Detection Capability
The wide tunability range of the solid-state laser enabled the PAS system to detect multiple gases simultaneously with high accuracy. By leveraging the laser's ability to cover the absorption spectra of both water and ammonia, the system demonstrated dual-gas detection capabilities using a single laser source. This capability is crucial for applications requiring comprehensive gas analysis and monitoring.
Stability and Reliability
The long-term stability testing of the solid-state laser source revealed consistent performance in terms of wavelength and output power. The low standard deviations observed for both parameters underscored the reliability of the laser source in maintaining stable operation, essential for accurate and reproducible gas detection.
Comparison with Conventional Methods
Compared to traditional gas detection techniques, the solid-state laser-based PAS system showcased significant advantages in terms of sensitivity, selectivity, and dual-gas detection capabilities. The precise tuning of the laser wavelength to target gas absorption lines, coupled with optimized experimental conditions, resulted in enhanced detection performance and improved analytical accuracy.
Conclusion
To sum up, this successful implementation of the solid-state laser-based PAS system for dual-gas detection opens new possibilities for advanced gas sensing applications. The system's high sensitivity, stability, and dual-gas detection capability make it a promising tool for environmental monitoring, industrial safety, and medical diagnostics. Further research and development in this area could lead to the integration of solid-state lasers into a wide range of gas sensing technologies, driving innovation in the field of trace gas detection.
Journal Reference
Qiao, S., He, Y., Sun, H., et al. (2024). Ultra-highly sensitive dual gases detection based on photoacoustic spectroscopy by exploiting a long-wave, high-power, wide-tunable, single-longitudinal-mode solid-state laser. Light Science & Applications 13, 100. https://doi.org/10.1038/s41377-024-01459-5, https://www.nature.com/articles/s41377-024-01459-5